Electron discharge apparatus



Dec. 18, 1956 c, BURKE 2,774,869

ELECTRON DISCHARGE APPARATUS Filed Nov. 7, 1950 2 Sheets-Sheet 1 Inventor P R E CBURKE Dec. 18, 1956 P. F. c. BURKE ELECTRON DISCHARGE APPARATUS 2 Sheets-Sheet 2 Filed NOV. 7,1950

PETER l T guake nited States Patent 2,774,869 ELECTRON DISCHARGE APPARATUS Peter Francis Conway Burke, London, England, assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application November 7, 1950, Serial No. 194,540

Claims priority, application Great Britain November 8, 1949 4 Claims. (Cl. 25027) The present invention relates to frequency multiplier apparatus for use at decimeter and centimeter wavelengths, especially where a largebandwidth is required.

Electron-velocity modulation'has in the past been proposed for use with frequency multipliers: thus in the klystron type of velocity modulated tube an input resonator may be used to modulate the velocity of a beam of electrons at a fundamental frequency, while an output resonator, tuned to a harmonic of the fundamental, is excited by the electron bunches resulting from drifting of the velocity modulated beam. Such apparatus, however, sufiers from the defect that, due to the necessarily sharp tuning of the cavity resonators, a fundamental carrier wave which is amplitude or frequency modulated 'over "a wide band cannot conveniently be employed.

It is known that apparatus such as the Kompfner travelling wave tube is capable of giving amplification over a very wide band. In such apparatus a slow-waveguide structure, such as a helix, is made to provide a component of the input signal travellingat a velocity near that of a beam of electrons which may be caused to travel along the same path as the said slow component. Such slow-Wave structures may be made dispersive so that the velocity of the slow wave varies with frequency.

The present invention uses apparatus in which a density modulated beam of electrons is introduced into travelling wave apparatus so dimensioned as to be sufficiently dispersive to discriminate between one particular harmonic and the others, including the fundamntal, but which, nevertheless, has a considerable bandwidth in the neighbourhood 0f the desired harmonic. This desired harmonic will be that for which the slow-wave velocity is the same as the beam velocity and continued interaction between the density modulated beam and the slow-wave structure.

will lead to its amplification in preference to the other harmonics. Thus, in this apparatus a wide band travelling wave structure is used in place of a high Q resonant standing-wave structure to extract the required harmonic from a density-modulated beam, and amplification of the required harmonic and not of the driving fundamental follows.

According to the present invention therefore, there is provided electron discharge apparatus comprising an electromagnetic waveguide adapted to propagate waves of a given frequency with a phase velocity a fraction that of light, means for projecting a beam of electrons along the said guide to progressively amplify waves of the said frequency, the dispersion of the Waveguide being such that the amplification is selective for that frequency with respect to sub-multiples and multiples thereof, and means for modulating the charge density along the beam at a sub-multiple of the given frequency in a region prior to the entrance of the beam into the said guide.

The means for modulating the charge density along the beam may take various forms, for example, another travelling wave structure may be used or a launcher resonator, as in a klystron, or, as is preferred, a cathodegrid structure, partof the electron gun, may be used as 2,774,869 Patented Dec. 18, 1956 in a class C amplifier or oscillator. As has been pointed out above, the travelling wave structure used in apparatus according to the present invention must be dispersive and, in general, special types of electron discharge device are required for use with the invention.

The invention also provides, therefore, an electron discharge device comprising an electron gun adapted to project a beam of electrons along the axis of a helix of conducting material dimensioned to propagate axial waves of "a given frequency with a phase velocity a fraction that of light, the propagation constant of the said waves and the mean radius a of the helix being such that the product a lies between 0.6 and 1.4, the said electron gun having a cathode and a control grid which, together with a beam accelerating electrode, would form, were the said accelerating electrode non-permeable to electrons, a triode amplifier valve suitable for use in a class C amplifier at a su'b multiple of the said given frequency, the said submultiple being the driving frequency it is desired to multiply. 1

Embodiments of the invention will be described with reference to the accompanying drawings, in which:

Fig. l is a diagrammatic representation, in part section of an electron discharge device connected in circuit according to the present invention;

Fig. 2 shows a modification of the apparatus of Pig. 1 using a cavity resonator as a buncher;

Fig. 3 shows an embodiment using a further travelling w-ave structure for bunching;

Fig. 4 shows, in part sectional elevation a further embodiment of the invention, and

Fig. 5 shows a plan view of part of the construction of Fig. 4.

In the various embodiments to be described, features common to different embodiments will be designated by like reference numerals. In the embodiment of Fig. 1 reference numeral 1 indicates an electron discharge device of the Kompfner travelling wave type, having a conventional helix 2 and collector electrode 3, surrounded by magnetic beam concentrating coils 4 and 5. A conventional type of waveguide output 6, together with an adjustable matching section 7, is connected to the havelling-wave tube in known manner. however, differs considerably from that conventionally used and consists of a cathode 8 and grid 9 similar to those in a U. H. F. triode. An accelerating anode 10, annular in shape so as not to intercept any of the beam, takes the place of the ordinary anode in the triode. Grid and cathode electrodes are brought out coaxially as indicated at -11 and 1-2 respectively, the resulting transmission line being tuned by the annular piston 13, and

the input is applied through the coaxial connection 14 from a source 15.

The helix 2 must be dimensioned so that the velocity of propagation of axial waves of the frequency of the source 15 differs from those of the sub-harmonics and harmonics of that frequency. It is known that the ratio of the apparent velocity of a wave along the wire of the helix to the velocity of light is a function of a where 'y is the slow wave propagation constant and a is the radius of the helix. For the product 'ya l.5 the helix is dispersive. For 'ya=1 at the frequency of the third harmonic the velocities for fundamental, second, third and fourth harmonics are in the ratio 2:l.6:l.4:1.2. It will be seen, therefore, that a helix can provide sufiicient discrimination to extract the third harmonic of an exciting fundamental. For satisfactory operation in apparatus according to the present invention, the product a for the helix 2 should lie between 0.6 and 1.4. Low values of 7 lead to a greater discrimination between har- 'monics but a smaller bandwidth, so the value chosen.

The electron gun,

should represent the desired compromise between very good harmonic selection and very wide bandwidth.

The member 11 connected to the control grid 8 of the electron gun may conveniently be earthed, as indicatedin the drawing, while the member 12 is connected through a cathode resistance 16 to the negative pole of potential source 17. A bias potential source 18 is shown connected between the grid member 11 and the junction of resistance 16 and source 17. The positive pole of source 17 is shown connected to the collector electrode 3 and a current source for heating the cathode 8' is connected at 19 to heater leads 20 brought out coaxially with the cathode conductor 12. Current for the focussing coils 4 and is provided by the source 2i. No connection to the input end of the helix is shown on Fig. 1. Although it may be connected to the accelerating anode 10, it is preferable to provide separate polarising potentials to the accelerating anode, the helix and the collector electrode.

As has been indicated above, the cathode, control grid and accelerating anode of the electron gun are arranged so that were a member non-permeable to electrons to replace the accelerating anode 10, the structure would form a triode suitable for use as a class C amplifier at the frequency of the driving source In the present apparatus the high frequency op'eratioi'r differs somewhat from that of the ordinary class C triode in the following manner.

The effect of transit time on the operation of the cathode-grid structure at high frequencies will be to shorten the angular duration of the current pulse passing the grid for a given angle of flow at the cathode. Since the anode is at a constant voltage, there will be no subsequent lengthening of the current pulse as happens in the class C operated triode at high frequencies. The radio frequency power needed to drive the grid, however, will increase, since the electrons emitted While the grid is positive, but which are later returned to the cathode because they have not passed the grid before it becomes negative, will absorb power without contributing to the current of the beam. This increase of radio frequency drive power and the difficulties of focussing the beam through the helix 2 at very short wavelengths give a limit for the maximum output frequency obtainable with the apparatus, output frequencies of 10,000 mcL/si being obtainable, however, with carefully designed embodiments of the invention.

If the drive power required for a particular angle of flow and average current is to be kept as small as possible, it is necessary that the transit angle of electrons from cathode to grid be small and that the transco'nductance be high. Both of these are achieved by bringing the grid as near the cathode as is mechanically possible, so that a spacing of about 0.005" is required for tubes driven with frequencies in the decirnet'er wavelength range. The distance from the grid to the accelerating electrode is determined partly by the need to keep the positive grid characteristics good, i. e. to reduce the current intercepted by the grid when it is positive. Finally the amplification factor chosen (which determines the pitch of the helix Winding) depends 'on the current desired at zero bias, the amount of negative bias required when operating under class C conditions and such factors.

In Fig. 2 of the accompanying drawings the class C triode electron gun of Fig. 1 is replaced by a cavity resonator 22 and an ordinary type of electron gun 23.

The resonator may conveniently be constructed as a pair of discs 24 sealed through the glass envelope 25 of the discharge device and clamped between metal rings 26, 27 and 28. The body of the resonator is connected to ground and a potential source 29 is connected between ground and the collector electrode 3. The helix is preferably polarised through separate connections'no't-s'ho'wn. A magnetic focussing coil 30 serves to focus the'beam from the electron gun through the resonator 22 and is supplied with current from D. C. potential source 31'.

The cathode of gun 23 is held at a negative potential with respect to ground by means of D. C. potential source 32. If desired, in order that the electron beam may be well bunched before entering the helix 2, a drift tube 33 may be inserted as shown between the helix 2 and resonator 22, the drift tube being connected to the end of the helix. 7 V

In place of the cavity resonator 22 of Fig. 2, a further travelling wave structure as shown in Fig. 3 may be employed. This embodiment comprises a helix 34 provided with conventional input and output arrangements, as indicated by the wave guides 35 and 36 respectively, wave guide 6 including a matching terminal indicated at 7.

Leakage of energy towards the electron gun or towards the helix 2 is prevented by means of quarter wave sleeves 42 and 43 terminating the helix and effectively short circuiting the apertures through the walls of the respective waveguides 35 and 36. A potential source 10 is shown connected between the cathode of the electron gun 23 and the helix 34, while polarising potential for the helix 2 is obtained by the source 4-1 connected between cathode and collector 3'. Alternatively separate polarising potentials may be applied to the helix and the collector.

If desired, instead of feeding beam modulating energy from the source 15, the modulating means may be arranged as an oscillator oscillating at the required subharmonic frequency. Thus, in Fig. 3, the source 15 and terminating arrangements 37 may be omitted and a feedback coupling provided by the waveguide 44, indicated by the dotted lines, a cavity resonator 45 being included in the feedback path if desired.

If it be preferred to use an oscillatory structure for density modulating the beam, the structure shown in Figs. 4 and 5 may conveniently be used. In this construction a resonant structure 46', functioning in the manner of a known coaxial line type of velocity modulation oscillator, is employed. The resonant structure shown comprises end discs 47 and 48 sealed respectively to envelope portions 49, surrounding the electron gun 23, and portion 50, surrounding the helix 2. A cylindrical metal wall 51 joins the two end discs 47 and 48. A central conductor 52 extends diametrically across the cylindrical member 51 and is apertured to permit passage of the electron beam. Pins 53, 54 and 55 are shown connected to the end discs47, 48 and to the central conductor 52, respectively, to define interaction gaps within the resonator, the passage through the conductor 52 forming a drift space between them. A focussing coil 56 is shown surrounding the helix and oscillator portions'of the tube.

Although in the embodiments described a helix is used for the dispersive waveguide, applications of the invention are not limited to this. Other forms of slowvelocity dispersive waveguides, such as two helices, one inside the other, a waveguide or coaxial line inductively loaded with slots or one in which the slots are detuned to give a flat phase velocity/frequency characteristic over a wide band with good rejection outside that band could also be used.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description'is made only by way of example and not as a limitation on the scope of the invention.

What I claim is:

1. Electron discharge apparatus comprising an electromagnetic waveguide in the form of a helical conductor, for propagating waves of a given frequency with a phase velocity a fraction that of light, the mean radius a of the helix and the propagation constant 'y' of the waves being such that the product 7a" liesb'etween 0.6 and 1.4, an electron beam source positioned to project a beam of electrons alongthe said guide to progressively amplify waves of the said frequency, the dispersion of the waveguide being such that the amplification is selective for that frequency with respect to sub-multiples and multiples thereof, a source of waves of a submultiple frequency of said given frequency positioned intermediate said beam source and said guide and means for modulating the charge density along the beam with energy from said wave source.

2. Apparatus according to claim 1, in which said helix has one end adjacent said electron beam source, the sole coupling to said one end being provided by said electron beam.

3. Electron discharge apparatus comprising an electromagnetic waveguide in the form of a helical conductor for propagating waves of a given frequency with a phase velocity a fraction that of light, the mean radius a of the helix and the propagation constant 7 of the waves being such that the product 'ya lies between 0.6 and 1.4, an electron beam source positioned to project a beam of electrons along the said guide to progressively amplify waves of the said frequency, the dispersion of the waveguide being such that the amplification is selective for that frequency with respect to sub-multiples and multiples thereof, a source of waves of a submultiple frequency of said given frequency positioned intermediate said beam source and said guide, and means for modulating the charge density along the beam with energy from said wave source, said beam source comprising an electron gun having a cathode and a control grid, said means for modulating comprising a transmission line surrounding said cathode and control grid and coupling said wave source between said cathode and control grid, and means for biasing said grid to project said beam only at the positive peaks of the modulating wave from said source.

4. Electron discharge apparatus comprising an electromagnetic wave guide in the form of a helical conductor for propagating waves, an electron beam source positioned to project a beam of electrons along said guide to progressively amplify said waves, a source of waves positioned intermediate said beam source and said guide, and means for modulating the charge density along the beam with energy from said Wave source, said beam source comprising an electron gun having a cathode and a control grid, said means for modulating comprising a transmission line surrounding said cathode and control grid and coupling said wave source between said cathode and control grid.

References Cited in the file of this patent UNITED STATES PATENTS 2,233,779 Fritz Mar. 4, 1941 2,300,052 Lindenblad Oct. 27, 1942 2,516,944 Barnett Aug. 1, 1950 

